Add basic 1-pass rate control

[barrbrain]

This is an adaptation of tterribe/rate_control8:

This is just the minimal patch required to get something working. There are still plenty of TODOs left to do: two-pass support, VFR support, golden frame support, the ability to drop frames, trial encodes for the first frames, quantization matrices, separate quantizer choices for Cb and Cr, proper lossless handling, support for AV1's levels, and some general refactoring to improve maintainability.

[barrbrain]

Preserved Tim's style for new code in src/rate.rs and matched style with the local context otherwise.

[coveralls]

Coverage increased (+0.9%) to 79.927% when pulling 5140ce8 on barrbrain:rate-control into 9e4ea86 on xiph:master.

[lu-zero]

Please rebase it.

[barrbrain]

With the encode-decode test, coverage for rate.rs is pretty good. Could possible pick levels carefully to hit more blocks.
234 of 286 new or added lines in 5 files covered. (81.82%)

[lu-zero]

I'd merge it now and refine later :)